(1) Field of the Invention
The present invention relates to non-aging low carbon, columbium-treated steel having no yield point elongation in the annealed condition, which has excellent surface characteristics and substantial freedom from non-metallic inclusions and a wide spectrum of mechanical properties, and to a method for producing the steel. While the term columbium is used herein, it should be understood that niobium is the same element. Although not so limited, the steel of the present invention in the form of sheet stock has particular utility in deep drawing and stretching operations, in metallic coating processes, and in the production of enameled steel.
(2) Description of the Prior Art
Both carbon and nitrogen give rise to yield point elongation in low carbon steels which have been recrystallization annealed, but strain aging which results in a return of yield point elongation after temper rolling in such steels is usually due to nitrogen. Such strain aging is prevented by adding aluminum which eliminates nitrogen from solution by formation of aluminum nitride. If aluminum stabilized steels are subjected to high temperatures after temper rolling, carbon will cause strain aging unless it also is removed from solid solution. Early workers in the art have stated that elements such as titanium, columbian, vanadium, zirconium, and chromium, if added in sufficient amounts to combine with all the carbon present in the steel, will eliminate aging and yield point elongation. Such elements have a strong affinity for carbon and form stable carbides, thereby removing soluble carbon from ferrite to such a low level that the as annealed yield point elongation is eliminated and strain aging is eliminated as well. The literature has indicated generally that the effectiveness of such elements in preventing aging increases with increasing affinity for carbon in the order -- chromium, zirconium, vanadium, columbium and titanium. See Journal of Iron and Steel Institute, 142, pages 199-221 (1940); Iron and Steel, June 1963, pages 326-334.
Thus, titanium has been considered the most effective element in eliminating aging and yield point elongation in low carbon steels, with columbium considered almost as effective, and other elements such as vanadium and chromium considered somewhat less effective. U.S. Pat. No. 3,183,078, issued May 11, 1965, to T. Ohtake et al., discloses a process for producing non-aging enameling iron having good drawability. This process involves producing a molten steel containing less than 0.04% carbon and an analysis otherwise comparable to conventional rimmed steel (except for a preferred manganese content of 0.05% maximum), vacuum degassing the molten steel to reduce the carbon content to less than 0.02%, less than 0.020% sulfur and 0.002 to 0.007% nitrogen, adding aluminum and titanium in amounts sufficient to combine with the carbon, nitrogen and sulfur present in the steel. In the preferred practice some aluminum is added first in order to combine with residual oxygen and nitrogen, thereby making most of the titanium available for combination with carbon, sulfur and any residual nitrogen not combined with aluminum.
French Pat. No. 1,511,529 granted Dec. 18, 1967, to Yawata Iron and Steel Co. Ltd. (the assignee of the above mentioned U.S. patent) discloses a process similar to that of the U.S. patent for the production of cold rolled sheet stock having good deep drawing and stretching properties. In the process of this French patent a molten steel is subjected to vacuum degassing with the addition of aluminum as a deoxidizing agent to produce a degassed steel containing less than 0.020% carbon and less than 0.015% oxygen. Titanium is added in a weight ratio of 4:1 to the carbon, and the degassed steel is then cast, hot rolled with a finishing temperature above 780.degree. C (1053.degree. K), cold rolled at a reduction rate above 30%, and finally annealed at a temperature between 650.degree. and 1000.degree. C (923.degree. and 1273.degree. K). The resulting sheet stock is stated to have a strong {111} orientation normal to the sheet surface, or cube-on-corner texture, and to have a plastic strain ratio (r value) ranging from about 1.75 to 2.47 depending on the processing used. The ASTM grain size ranges from 7.5 to 10.
The r values set forth in the Yawata French patent are not identified as to which r value is designated. In any event, titanium-bearing steels produced by similar processing by applicants and others in the United States indicate that average r values above about 2.0 cannot be obtained.
In the present application the average plastic strain ratio r is the standard calculated as EQU r = 1/4[r(longitudinal) + r(transverse) + 2r(diagonal)].
While the addition of titanium to a vacuum degassed steel results in a product having non-aging properties and no yield point, the product nevertheless suffers from a number of disadvantages. Since titanium is a strong nitride, oxide and sulfide former, as well as a carbide former, a larger addition of titanium than the amount theoretically necessary to combine with carbon is required because of the reaction of part of the titanium with nitrogen, oxygen and sulfur present in the steel. Thus, although the theoretical stoichiometric ratio of titanium to carbon is about 4:1, this must be increased initially to a ratio of about 8:1 because titanium reacts with the residual sulfur and nitrogen in the steel. In addition, still more of the titanium is lost as a result of titanium oxide formation which goes into the slag. It has therefore been found that in commercial practice titanium must be added in a weight ratio to carbon of as high as 16:1 in order to obtain a non-aging steel having no yield point. The titanium recovery may thus be on the order of 50 to 60% under such circumstances.
The formation of oxides, nitrides and sulfides of titanium in the steel results in objectionable non-metallic inclusions of these compounds and adversely affects the surface quality of the product.
Titanium in solution in the steel may prevent the healing of hot cracks, as is known to be the case with aluminum.
The great affinity of titanium for oxygen in the air also renders the molten steel less fluid during casting.
Moreover, the titanium bearing steels of the type disclosed in the above mentioned French patent have inherently low strength, not exceeding about 20,000 psi yield strength (138 MN/m.sup.2), which cannot be increased substantially by the final annealing treatment.
Due to the above disadvantages and to the increased cost resulting from the practical necessity of adding up to four times the theoretical amount of titanium needed, vacuum degassed, titanium-treated steels have not gained commercial acceptance over rimmed and killed steels for deep drawing, stretching, coating, or enameling applications.
It has previously been reported by Abrahamson et al. in "Transactions Metallurgical Society of AIME", VOl. 218, December 1960, pages 1101 - 1104, that columbium and zirconium substantially retard the rate of recrystallization during annealing of cold rolled material in comparison to alloying elements such as titanium and chromium. These findings were based on one-hour anneals with increasing temperatures throughout each anneal. However, no practical benefit or advantage has ever previously been derived from this knowledge.